What is the purpose of the proportioning valve in the hydraulic braking system of a front-engine rear-wheel drive vehicle?

1. To increase the braking pressure at the rear wheels under heavy loads.
2. To limit the hydraulic pressure sent to the rear brakes during hard stops to prevent rear wheel lockup due to forward weight transfer.
3. To divide the master cylinder pressure equally between all four wheels under all conditions.
4. To assist the driver by increasing pedal force dynamically.